The main objective of this project is to test alternative hypotheses:
- That there is a common basal dentition characterizing all crown group gnathostomes (Osteichthyes + Chondrichthyes)
- That patterned dentitions evolved separately in each group.
This is one of the key questions in vertebrate evolution; how did these two major jawed vertebrate groups build the dentitions so responsible for their evolutionary success? Testing these hypotheses will provide a context for interpreting stem-osteichthyan and stem-chondrichthyan dentitions but also, ultimately, for interpreting dentitions of stem-gnathostomes (including 'Placodermi'). Currently, stem-chondrichthyans include problematic taxa previously assigned to the fossil group 'Acanthodii', although recently some acanthodians were identified as stem-osteichthyans.
Patterns of dentition development and associated gene expression have been investigated in only a small number of crown-group gnathostomes including the small-spotted catshark (Chondrichthyes), the Queensland lungfish, trout, cichlids and pufferfish (Osteichthyes). The latter three are derived osteichthyans (Teleostei), with substantial phylogenetic gaps between all these taxa. These gaps make an assessment of a basal crown-gnathostome dentition problematic. To fill this gap, we will focus primarily on the chondrichthyan dentition, including groups with superficially different dentitions: Batoidea (skates, rays) and Selachii (sharks). Among the osteichthyans, we will focus on more phylogenetically basal non-teleost taxa, Polyodon (paddlefish) and Lepisosteus (gar).
We will collate morphological, developmental and molecular characters for these taxa, from teeth in both oral dentitions and those in extra-oral clusters. Each will form a separate component of this grant, requiring different techniques, but with the overarching goal of testing our two hypotheses.
A) Examine ontogenetic series of dentitions in extant crown-group gnathostomes, for common patterns among the major clades Chondrichthyes and Osteichthyes.
Chondrichthyes: We wil examine ontogenetic series of extant batoid and selachian dentitions (Neoselachii), to discover common patterns of development:
1) presence or absence of a symphyseal set of teeth; 2) alternate tooth addition versus addition in single-family sets; 3) lifetime proximal tooth family addition versus early establishment of a fixed number; 4) abrupt morphological changes within dentitions; 5) differences in morphological characters between upper and lower jaw dentitions; 6) developmental order in extra-oral (pharyngeal) denticles; 7) development of rostral 'teeth', with replacement, or with continuous growth.
These characters will be compared in a diverse suite of extant Neoselachii from all three major clades, extinct Neoselachii (Synechodus, Sclerorhynchoidea), Hybodontidae, sister group to Neoselachii; stem-chondrichthyans, especially Cladoselache.
Osteichthyes: We will examine ontogenetic series of the extant paddlefish Polyodon (Chondrostei; Actinopterygii) and the gar Lepisosteus (Holostei; Actinopterygii), representative of phylogenetically basal Osteichthyes, to establish any dentition characters in common with Chondrichthyes. The bichir (Polypterus) and bowfin (Amia) will also be examined.
B) An 'ancient gene network' (AGN) for gnathostome dentitions, both oral and pharyngeal (associated with the pharyngeal arches), has recently been established. Preliminary work on the shark Scyliorhinus canicula has shown that shh expression is involved in tooth induction, determining alternate positions and the number of tooth families along the jaw shh also determines tooth position in teleosts, and in the cichlid jaw, edar, shh, pitx2, eda, and wnt7b are involved in spacing of teeth and of tooth sets. We will consolidate comparable details of gene expression in chondrichthyans such as Scyliorhinus and Leucoraja, and the phylogenetically basal osteichthyans Polyodon and Lepisosteus.